linux-kernel - Re: [PATCH v9 12/17] mm: move lesser used vma_area

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Message-ID: <20250115105056.GY5388@noisy.programming.kicks-ass.net>
Date: Wed, 15 Jan 2025 11:50:56 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: akpm@...ux-foundation.org, willy@...radead.org, liam.howlett@...cle.com,
	lorenzo.stoakes@...cle.com, david.laight.linux@...il.com,
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	linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v9 12/17] mm: move lesser used vma_area_struct members
 into the last cacheline

On Fri, Jan 10, 2025 at 08:25:59PM -0800, Suren Baghdasaryan wrote:
> Move several vma_area_struct members which are rarely or never used
> during page fault handling into the last cacheline to better pack
> vm_area_struct. As a result vm_area_struct will fit into 3 as opposed
> to 4 cachelines. New typical vm_area_struct layout:
> 
> struct vm_area_struct {
>     union {
>         struct {
>             long unsigned int vm_start;              /*     0     8 */
>             long unsigned int vm_end;                /*     8     8 */
>         };                                           /*     0    16 */
>         freeptr_t          vm_freeptr;               /*     0     8 */
>     };                                               /*     0    16 */
>     struct mm_struct *         vm_mm;                /*    16     8 */
>     pgprot_t                   vm_page_prot;         /*    24     8 */
>     union {
>         const vm_flags_t   vm_flags;                 /*    32     8 */
>         vm_flags_t         __vm_flags;               /*    32     8 */
>     };                                               /*    32     8 */
>     unsigned int               vm_lock_seq;          /*    40     4 */

Does it not make sense to move this seq field near the refcnt?

>     /* XXX 4 bytes hole, try to pack */
> 
>     struct list_head           anon_vma_chain;       /*    48    16 */
>     /* --- cacheline 1 boundary (64 bytes) --- */
>     struct anon_vma *          anon_vma;             /*    64     8 */
>     const struct vm_operations_struct  * vm_ops;     /*    72     8 */
>     long unsigned int          vm_pgoff;             /*    80     8 */
>     struct file *              vm_file;              /*    88     8 */
>     void *                     vm_private_data;      /*    96     8 */
>     atomic_long_t              swap_readahead_info;  /*   104     8 */
>     struct mempolicy *         vm_policy;            /*   112     8 */
>     struct vma_numab_state *   numab_state;          /*   120     8 */
>     /* --- cacheline 2 boundary (128 bytes) --- */
>     refcount_t          vm_refcnt (__aligned__(64)); /*   128     4 */
> 
>     /* XXX 4 bytes hole, try to pack */
> 
>     struct {
>         struct rb_node     rb (__aligned__(8));      /*   136    24 */
>         long unsigned int  rb_subtree_last;          /*   160     8 */
>     } __attribute__((__aligned__(8))) shared;        /*   136    32 */
>     struct anon_vma_name *     anon_name;            /*   168     8 */
>     struct vm_userfaultfd_ctx  vm_userfaultfd_ctx;   /*   176     8 */
> 
>     /* size: 192, cachelines: 3, members: 18 */
>     /* sum members: 176, holes: 2, sum holes: 8 */
>     /* padding: 8 */
>     /* forced alignments: 2, forced holes: 1, sum forced holes: 4 */
> } __attribute__((__aligned__(64)));